FIELD OF THE INVENTION
The invention relates generally to a method for safeguarding the discharge of residual heat from a reactor of a nuclear power station in the case of a lowered filling level in a primary circuit of a reactor cooling system. In particular, the method is to be employed during cleaning and maintenance operations in nuclear power stations having a pressurized water reactor. Moreover, the invention also relates to a device for safeguarding the discharge of residual heat from a reactor of a nuclear power station.
In nuclear power stations, particularly those with pressurized water reactors having a western-type of construction and mode of operation, the so-called discharge of residual heat below specific pressure and temperature states in the reactor cooling system takes place through the primary side of the cooling system. In methods according to the prior art the following method steps are carried out when maintenance and cleaning operations are initiated in the reactor:
Initially, the reactor is shut down and there is a wait until the cooling liquid in the primary circuit has cooled. Moreover, the pressure in the cooling system is reduced. PA1 Then, the filling level of the primary circuit is lowered to about a mid-loop level of an MC conduit (main coolant conduit), an aftercooling system is cut in which takes over the heat discharge from the primary circuit, and the heat discharge in that opened and lowered state is no longer provided by a steam generator plant which is otherwise connected to the primary circuit when the nuclear power station is in operation. PA1 Then, as a rule, in nuclear power stations of the type in question, a coolant reservoir is provided for the required refilling of the primary circuit of the shut-down reactor and/or of the aftercooling system. The coolant reservoir can be a boron water storage tank which is provided within the reactor containment and which is designated as "IRWST" (Incontainment Refilling Water Storage Tank). Such a coolant reservoir serves, as a rule, for flooding spaces above the opened reactor to the level of the fuel-assembly storage basin, until the fuel elements can be changed below the surface of the liquid. In European reactors of recent construction (EPR=European Pressurized Reactor), the coolant reservoir is provided as an "Incontainment System".
Such an emergency cooling plant with a coolant reservoir can be taken from a book entitled "Lexikon der Kern- und Reaktortechkik" [Lexicon of Nuclear and Reactor Engineering], Franckh'sche Verlagshandlung, W. Keller & Co., Stuttgart, Germany, 1959, pages 142-144.
An emergency cooling system which has a water tank is also known from U.S. Pat. No. 5,268,943. Cooling water originating from that water tank can be fed, if necessary, to the circuit of a nuclear reactor. Furthermore, the tank can then be refilled, when there is sufficient water in the circuit.
During the entire phase of maintenance work in which the filling level in the reactor cooling system is lowered, residual heat discharge, which is very important for the safety of a nuclear power station, may be appreciably disturbed by system errors. One of those possible malfunctions is an inadvertent excessive lowering of the filling level, which may lead to a total failure of aftercooling in the case of a relatively high decay potential of the reactor. In such an instance, the aftercooling pumps of the cut-in cooling system draw in air, so that bubbles may form in the cooling system, with the result that the cooling capacity falls rapidly.
While maintenance work is being carried out, as well as while in a pressureless state, the cooling system is also evacuated for coolant degassing in the case of a lowered filling level. As a result, with the filling level lowered, two different pressure states are to be observed in the reactor cooling system and on the cooling systems connected to it, and those pressure states make it even more complicated to limit and rectify malfunctions.